Basic ikkyo technique from a punch. Adapting to an opponent with a rigid arm. Use atemi to break uke's posture (pic 2), Don't fight the rigid arm (pic 3). Let it work against uke and compromise his posture (pic 4). Control uke's arm with a nikyo grip, keeping wrist above shoulder and step into uke, breaking his balance again (pic 5). Step away from uke, breaking their balance and bringing them to the mat (pic 6).